Prior Art Statement
The following patents are representative of the most relevant prior art known to the Applicant at the time of filing the application.
______________________________________ U.S. Pat. Nos. 2,271,362 January 27, 1942 T. E. Field FOREIGN PATENTS AND PUBLICATIONS 621,736 April 19, 1949 United Kingdom 2,478,621 September 25, 1981 France ______________________________________
The Field patent, U.S. Pat. No. 2,271,362 is concerned with cast chrominum oxide based refractory products. Chromic oxide and up to 21% by weight of magnesium oxide in granular or powdered form are fused together and the molten fusion product is cast into the desired shape such as, for example, bricks, plates, and the like. Raw materials such as chrome green pigment type of chromic oxide, and magnesite can be employed. Field is relevant for its general teaching of chromia-magnesia refractories and the utilization of chrome green oxide and magnesite as starting materials.
U.K. Pat. No. 621,736 discloses another chromite based cast refractory composition which is particularly suitable for use in electric furnaces for melting alloy steels wherein the slag produced contains high amounts of iron oxide and alumina but low amounts of alkaline earth oxides such as magnesia and calcia. The fusion of Cr.sub.2 O.sub.3, FeO, Al.sub.2 O.sub.3 and MgO.Al.sub.2 O.sub.3 which is highly stable with respect to the particular type of slag involved.
The most relevant reference is French Patent Publication No. 2,478,621. Unlike the other two references, this reference is directed to chromia-magnesia based refractory products wherein these refractory products are not fabricated by fusion of a mixture of chromia and magnesia and casting of the molten mixture in appropriately shaped molds. The products of the French publication are sintered products. Chromia 71 to 82% by weight, and magnesia 16 to 28% by weight, are fused to produce a fused product that is at least 94% chromia-magnesia. This product, in the shape of an ingot for example, is then crushed to grains having a maximum size of 12 mm. The reference, for example, teaches using fused, crushed chromia-magnesia with a particle size distribution of 25% of 6-12 mm grains, 25% of 1.5-6 mm grains, 20% of 1.5 mm and finer, and 30% of 0.12 and finer, to form a green refractory shape in the conventional manner. The green shape is then fired at 1750.degree. C. in a reducing atmosphere in order to sinter the afore described chromia-magnesia grain mixture. The resulting products varied in chromia-magnesia total content of from about 95% by weight to 99% by weight. The densities obtained by the reference ranged from 3.70 to a maximum of 3.77 Mg/m.sup.3. According to the French reference, the composition of the fused, crushed chromia-magnesia grains is preferably rich on the MgO side of the Cr.sub.2 O.sub.3 -MgO spinel composition, i.e. 79.2% Cr.sub.2 O.sub.3 and 20.8% MgO although as pointed out above, the reference discloses a broad range which includes compositions on the chromia rich side of the spinel formulation as well.
The present invention differs from both the Field and United Kingdom references in that these references are concerned solely with fused cast chromia-magnesia spinel refractory products, while the invention product is made up of grains of fused crushed chromia-magnesia bonded with an in-situ formed chromia or chromia-magnesia bond. The present invention differs from the French publication because the French reference teaches a product formed by sintering particles of chromia-magnesia spinel and not bonding said particles by a microcrystalline chromia-magnesia bond.